Automotive designers are directing considerable effort toward the development of NO.sub.x trap technology for lean burn automotive engines. A major engineering obstacle associated with the application of a NO.sub.x trap resides in the trap's rather limited temperature operating window. High NO.sub.x absorption efficiencies are achieved (greater than 80%) only when operation occurs within a very narrow temperature range, between 250-450.degree. C. As a result, if an NO.sub.x trap is positioned so as to provide good NO.sub.x absorption efficiency during periods of light engine speed/load conditions, the NO.sub.x trap temperature during heavier engine speeds and loads will be too high (i.e., greater than 450.degree. C.) for good NO.sub.x trap performance. If, on the other hand, the NO.sub.x trap is positioned farther away from the engine's exhaust manifold in order to provide for NO.sub.x trap operation within the desired range of 250-450.degree. C. during high speed/load conditions, the NO.sub.x trap temperature could be considerably less than 250.degree. C. during lighter engine speeds and loads.
The present invention provides a solution to the problem of NO.sub.x trap temperature control. Although it is known in the art to provide multiple flow passes for the exhaust gas so as to control the temperature of gases reaching an NO.sub.x trap, such systems, such as that disclosed in Japanese patent application 05-216603, utilize multiple valves to control the flow through the system. This is undesirable because cost and complexity are increased, while reliability is decreased. The present system permits governing of the temperature of exhaust flow reaching an NO.sub.x trap with a passive flow network, and without the need for externally controlled valves or other devices.